1. Field of the Invention
The present invention relates to a vehicle running condition judgment device, and in particular to a device for judging a road surface condition and a vehicle under limit condition, using a vehicle motion model.
2. Description of the Related Art
A road surface condition judgment device is mounted on a vehicle, and judges a road surface condition while the vehicle is running. A judgment result on the road surface condition obtained on such a device is used in electronic running control. For enhanced driving safety, judgment with improved accuracy on road surface condition has been desired. In particular, for various driving control in a four-wheel steering system and a vehicle spin prevention system, accurate comprehension of road surface condition is critical.
Japanese Patent Laid-open No. Hei 8-119131 discloses a device for detecting a slippery state of a road surface. Based on values obtained from various sensors (sensor values) and pre-stored tire characteristics substituted into a vehicle motion model (an equation of state), the device estimates reference state quantity (e.g., a vehicle lateral acceleration). Then, a difference (a residual error) between the reference state quantity and detected state quantity based on a sensor value (e.g., a vehicle lateral acceleration) is calculated, and a predetermined number of such data items on the difference are ordered in time series to form reference difference time series data. Meanwhile, a plurality of estimated difference time series data items are prepared according to respective road surface conditions. Then, the pattern of the reference difference time series data is compared with those of the plurality of estimated difference time series data items to see which data item shows the closest pattern to that of the reference difference time series data for judgment on current road surface condition.
Japanese Patent Laid-open No. Hei 9-311042 discloses a device for estimating a vehicle slip angle using a vehicle motion model, similar to the above. In this device, a lateral acceleration estimated from a cornering performance model is compared with a detected lateral acceleration for judgment on the road surface condition, based on which tire characteristics are determined. The determined tire characteristics are in turn used in estimation of a vehicle slip angle. For estimation of a vehicle slip angle, a feed back control method utilizing an observer is employed, details of which are also disclosed in Japanese Patent Laid-open No. Hei 3-122541. In this method, a state estimation technique, one of the modern control theories, is utilized in detection of a vehicle slip angle.
A conventional device disclosed in the above mentioned Japanese Patent Laid-open No. Hei 8-119131, however, aims at detection of slippery state of a road surface only at the initial stage of a vehicle's turning before spinning, and can therefore be used, in principle, for judgment on road surface condition only at the initial stage of a vehicle's turning. After all, this device can not be used when the road surface condition varies while the vehicle is turning, and the road surface condition in such circumstances can therefore not be detected. Specifically, in the above conventional device, phase deviation is caused, as the vehicle begins slipping, between a modeling result and an actual value obtained on a sensor. As such, the reference difference time series data matches to none of the estimated difference time series data, resulting in failure of accurate judgment on road surface condition. In other words, judgment on road surface condition by the above conventional device is achievable only at the initial stage of a vehicle's turning.
Also, various conventional devices are incapable of correct determination on road surface condition when a vehicle runs under various running conditions. For example, there has conventionally been a problem that judgment on road surface condition is applicable only at the initial stage of a vehicle's turning.